Quality Parameters of Audio Signals

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Wow and Flutter is a deformation of the original sound, which can only occur, if mechanical recording was involved. Therefore it is a problem, which only concerns tape and cassette machines and record players.

6.1. The Audible Effects of Wow and Flutter

Wow and flutter produces some of the most nasty deformations of sound. The typical impression should be known from poor cassette recorders. The listener experiences a change in frequency of the original signal.
The English terms "wow" and "flutter" describe the characteristic very well.

If it is a slow change (less than ten per second), we call it wow.

If it is a fast change (more than ten per second), we call it flutter.

Distinguishing wow and flutter is quite useful, because they often have different causes.

The list indicates, that wow and flutter problems are normally caused by lacking or wrong maintenance. Often it is not caused by some defect, but can be avoided by cleanliness and proper servicing of the equipment.

The operators and producers can contribute to the required cleanliness of the equipment by keeping coffee, tea, softdrinks and cigarettes away from it.

It should be noted, that all analog recording equipment will produce a small amount of wow and flutter. Under normal operating conditions this should be so little, that it is inaudible.

6.2. Definitions of Wow and Flutter.

Technically wow and flutter is a frequency modulation of the original signal. The frequency deviation defines the intensity of wow and flutter and the modulation frequency defines the frequency of wow and flutter.

To measure wow and flutter, a stable frequency is recorded. Then the frequency deviation is measured during play-back. The peak frequency deviation is related to the recorded frequency and the relationship is given in per cent.

The human ear has the highest sensitivity for wow and flutter at frequencies around 3kHz. The test frequency for wow and flutter is standardized by CCIR to 3150Hz. (3000Hz for NAB). For the purpose of wow and flutter measurement, test tapes and test records are available with this special test frequency.

Fig. 6.2.1.

Wow and flutter produce a frequency deviation of the original signal. The test signal is normally 3150Hz. The value of wow or flutter is determined by the ratio of peak value of the frequency deviation to test frequency.

The human ear recognizes frequency deviations differently, depending on the frequency at which it occurs. We will find a frequency deviation the most annoying, if it occurs with a frequency of 4Hz. Higher or lower modulation frequencies will be recognized less. Therefore wow and flutter is weighted with a frequency response, which simulates the sensitivity of the human ear for wow and flutter.

Fig. 6.2.2.

The weighting curve for wow and flutter according to CCIR 409-2. The curve has a maximum at 4Hz and is producing less sensitivity towards higher and lower frequencies.

Wow and flutter can only be measured with special wow and flutter meters. The meters contain a generator, producing the test frequency. The analysing circuit consists basically of a FM de-modulator, a weighting filter and an indication device.

When no wow and flutter test tape is available, a recording is to be done with the test signal (3150Hz) from the wow and flutter meter. Then the tape is to be reproduced, while the output signal from the tape machine is fed to the input of the wow and flutter meter. The meter will indicate the value. If the indication is fluctuating, the average value is considered.

Fig. 6.2.3.

The set-up for wow and flutter measurements:
First the test signal (3150Hz) is recorded on tape.
Then the tape is reproduced and the signal is analyzed by the wow and flutter meter.
Simultaneous recording and reproducing should be avoided.

Wow and flutter meters are also able to measure the speed drift (deviation).
Drift produces not a varying, but a constant change in frequency.
One can also say, that drift is a wow of very low frequency.

Normally the absolute deviation produce no negative audible effects. But it indicates some problem with the drive system of tape machines and record players. Therefore it is useful to measure it during maintenance.

Using the vary-speed of tape machines or record players will produce drift purposefully.

Quality Parameter of Audio Signals (530KB) Download this document in Acrobat format for printing and offline viewing.

Quality Parameters of Audio Signals